Malted barley is used to make beer, malt vinegar and other things. In my opinion, its highest and best use is achieved when it's added to a vanilla milkshake to make a vanilla malt -- YUM -- though I'm sure your cookies will be delicious too. For baking, I'm pretty sure you want diastatic malt powder or malt flour. I know they sell it at Whole Foods, and probably at other grocery stores as well.

I use diastatic malt powder when baking bread--it helps bread rise better. I order it from King Arthur Flour. They also sell non-diastatic malt powder, and that's most often used for making bagels. The difference between the two has something to do with enzymes--diastatic has them, and non-diastatic doesn't.

...and you can make your own by sprouting grain (must be whole, esp. barley), drying it, grinding it. I did it once, following an article in Prevention magazine, in the 70's,
Carnation used to sell malt powder that was intended for sodas. I don't think it's available now.